As the conventional methods for forming a complex or inclusion compound of a slightly water-soluble drug with a cyclodextrin, there are a method wherein a saturated aqueous solution of a drug and a cyclodextrin are cooled and the resulting complex is precipitated, a method wherein an aqueous solution of a drug and a cyclodextrin is lyophilized [M. Kurozumi et al., Chem. Pharm. Bull., 23, 1421 (1975)], a mixing and pulverizing method [Y. Nakai et al., Chem. Pharm. Bull., 26, 2419 (1978)] and the like. However, the complexes of a slightly water-soluble drug with a cyclodextrin obtained by these methods do not have so high water-solubility and their water-solubility is insufficient for injection preparations. Further, in the complexes, the stability of drugs are sometimes deteriorated.